The present invention relates to microwave filters of small size, comprising linear resonators formed by one or more conductors.
It is known that band-pass or band cut-off microwave filters may be produced with resonators formed by U-shaped conductors deposited by metallization or any other equivalent means on a first plane surface of a substrate whose second surface, parallel to the first surface, is metallized in order to form a ground plane.
According to this arrangement, the branches of the U-shapes forming the resonators are mutually parallel and are dimensioned so that the total developed length of each of the U-shaped elements is equal to half the tuned wavelength .lambda. of the resonator.
The coupling factor between two resonators depends on the width of the conductor forming the resonator, on the distance which separates the branches of two adjacent U-shaped elements, as well as on the space existing between the two branches of one and the same U element.
The principal shortcomings of these filters are that they have parasitic responses at the multiple frequencies of their central operating frequency, in particular if they are situated within a closed casing, and that they have an appreciable bulk, mainly at frequencies lower than 8 GHz.
In order to overcome these disadvantages, it is commonly attempted to reduce the dimensions of the casings by reducing the dimensions of the resonators. For example, one solution consists in placing a capacitor between the free extremities of the branches of the U-shaped element of each resonator in order to tune the same to its operating frequency. This embodiment equally has as the advantage that it makes it possible to obtain filters having a satisfactory rejection of the parasitic frequencies. However, it has the disadvantage of giving rise to substantial electrical fields at the level of the capacitors and parasitic couplings between non-adjacent resonators which impair the response of the filter. Because of this, the physical behavior of a filter produced in this manner never corresponds to that of the filter to be expected theoretically, but to an approximation which on the one hand requires several long and careful tests for its production, and on the other hand, as a corollary, increases the cost price.